Lead sulfide photoconductive cell



June 21, 1955 N. c. ANDERSON ETAL 2,711,464

LEAD \SULFIDE PHOTOCONDUCTIVE CELL Filed Nov. 28, 1952 FIG? IN VEN T0195 N.C. ANDERSON 5.5. SHAW ATTORNEY United States Patent LEAD SULFIDE PHOTOCONDUCTIVE CELL Norman C. Anderson, Auburndale, and Burton E. Shaw, Lynn, Mass assignors to Electronics Corporation of America, a corporation of Massachusetts Application November 28, 1952, Serial No. 323,115 4 Claims. (Cl. 201-63) This invention relates to a new and improved lead sulfide photoconductive cell.

A principal object of this invention is to minimize the susceptibility of lead sulfide cells to the deleterious effects of atmospheric gases and humidity.

Another object of this invention is to improve the operating stability of lead sulfide photoconductive cells.

Most of the practical photoconductive cell structures of the prior art have comprised a variety of photoconductive materials positioned within different types of hermetically sealed envelopes. Generally speaking, these envelopes were similar or the same as those utilized for enclosing the electrode structures of vacuum tubes. The photoconductive materials were either deposited or otherwise applied directly to the inner envelope walls, or they were supported by additional structure located within the envelopes. Simple electrodes or complex grid structures were used to make direct electrical connections to the photoconductive materials, and conventional feed-through pins sealed in the envelope walls provided the necessary external connections to the electrodes or the grid structures, as the case may be.

The fabrication of these photoconductive cells was relatively difiicult and expensive because of the complex and time-consuming procedures required. In particular, the preparation of the glass envelopes to receive the photoconductive materials, the positioning of the electrodes or grid structures within the envelopes, and the sealing of the feed-through pins to the envelope Walls could only be performed by skilled technicians. Moreover, the evacuating, filling, and hermetically sealing of the envelopes added considerably to the cost and time required to process these cells.

The prior art cells were also extravagant in the amount of material consumed in housing a relatively small amount of photoconductive material. That is, a relatively large cell envelope was always used to enclose a small area of photoconductive material of extreme thinness because of the ease of assembly within a large envelope and the consequent manufacturing economy. However, the relatively large size of these cells precluded their use in many applications for which they were otherwise advantageously suited.

Some of the objections to photoconductive cells of the hermetically sealed envelope type were overcome by a radically different structure. This structure comprised a solid glass disc having a complex grid applied thereto and upon which grid a thin coat of photoconductive material was deposited. The active surface surrounding the grid was sealed from harmful oxidizing vapors by a thin coat of paraflin. This assembly was coupled and interconnected to supporting means having conventional pins for mating with a socket.

This structure was relatively ditficult to fabricate, however, notwithstanding the absence of an envelope, because of the complex grid and the necessity for interconnecting the end terminals of the grid to the pins of the supporting iii) 2,711,464 Patented June 21, 1955 means. Furthermore, the size of this cell was unduly large because of the complex grid.

With the realization of the importance of lead sulfide as a photoconductive material, photoelectric cells having a solid body structure of a size approximately equal to that of small beads became feasible. In the co-pending application of N. C. Anderson et al., Serial No. 216,226, now Patent No. 2,674,677, a lead sulfide photoconductive cell is shown wherein this material is applied directly to the outside surface of a small bead, so that two electrode pins mechanically coupled therein are electrically interconnected. Inasmuch as the characteristics of lead sulfide vary to a great extent upon exposure to excessive humidity conditions, the effective isolation of this material upon its head-like body structure is of extreme importance.

While the structure described in the aforementioned application was admirably suited to conditions wherein atmospheric humidity and moisture condensation upon the cell was kept to a minimum, in environments wherein such conditions could not be met, moisture creepage along the surfaces of the bonded pins, notwithstanding they were imbedded in glass, and creepage beneath the protective covering and thence to the activated lead sulfide area minimized the utility of such cells.

Accordingy, the novel photoconductive cell hereof contemplates a structural arrangement somewhat analogous to that of the cited application, but differing in an important aspect in that the lead sulfide is applied to a small glass plate which is so suspended in an environment of resinous material as to prevent moisture creepage to the active area of the cell. Cells of this construction have been successfully immersed in a water bath for periods of several days and then have been removed and placed in an electric circuit with no appreciable change in the electrical characteristics noted.

In order that all of the features of this invention and the mode of operation thereof may be readily understood, a detailed description follows hereinafter, with reference being made to the drawings wherein:

Fig. 1 is a perspective view of a glass plate to which the activated lead sulfide is applied;

Fig. 2 is a side elevation view of the glass plate shown in Fig. 1;

Fig. 3 is a plan view of the body structure of the cell of this invention showing the initial application of silver paste and resinous material to the top surface;

Fig. 4 is a sectional view of the body structure of the cell of this invention with the electrode pins being shown in full;

Fig. 5 is a perspective view of a completely fabricated photoconductive cell of this invention;

Fig. 6 is a side-elevational view of the photoconductive cell shown in Fig. 5; and

Fig. 7 is an enlarged partial sectional view line 7-7 shown in Fig. 6.

Referring now principally to Figs. 5 and 6 wherein the general details of the novel cell hereof are shown, it will be noted that the cell comprises a glass or ceramic body 16 having a projecting disc-shaped upper portion and a cylindrical lower portion. This body construction is preferred, in that it facilitates manual removal of the cell from a socket. A set of two pins 14 passes through body 16 and makes electrical contact with an active area of lead sulfide which is deposited upon the underlying surface of glass plate 8. Electrically speaking, the cell hereof comprises the set of pins 14 whose non-exposed ends are interconnected by an active area of lead sulfide deposited on the underlying side of plate 8. The details of the structure providing this connection will be set forth hereinafter, particularly with reference to Fig. 7 of the drawing. Plate 8 is mechanically bonded to body 16 and also taken along isolated from the atmosphere by a protective coat of resinous material 15.

The cell is responsive to light energy impinging generally upon plate 8, as shown by the arrow of Fig. 5. This radiation is not substantially attenuated by the resinous protective covering and is readily transmitted through the width of plate 8 to the underlying area of photoconductive material. Pins 14, when inserted into a socket, provide the necessary electrical connections to the cell.

The details of plate 8 and its adhering materials are shown in- Figs. 1 and 2. In the initial fabrication of plate 8, a thin coat of lead sulfide 9 is deposited upon plate 8 and thereafter activated. Thereafter, two silver paste tabs 10 are applied to the end portions of lead sulfide coating 9. A setof aquadag bonding tabs 11 isapplied so as to completely cover the joining portions of silver paste tabs 10 and lead sulfide coating 9. The application of these low-resistance aquadag bonding tabs minimizes the generation of photovoltaic currents at the lead sulfide and silver paste junctions. The portion of lead sulfide coating 9 positioned between the set of aquadag bonding tabs 11 comprises the active photoconductive area of the cell hereof.

Referring to Fig. 4, in the initial construction steps of the cell hereof, the set of pin electrodes 14 is mechanically bonded to body 16 by solder glass or resinous material 18 which is fed through cup 17. The top circular face of body 16 is thereafter polished, so that the end portions of pins 14 are flush with this top face. A set of silver paste tabs 12 is applied to this top face so as to cover the top end portions of pins 14, as shown in Fig. 3. Upon the drying of these silver paste tabs, a thin layer of resinous material 13 is applied. Thereafter, this resinous material is also applied directly to the portion of lead sulfide surface 9 located between aquadag tabs 11. Likewise, additional silver paste is applied to the exposed portions of silver paste tabs 10, and glass plate 8 is then placed upon the prepared top surface shown in Fig. 3, so that tabs 10 mate and contact tabs 12. With the positioning, and after the setting of the silver paste, resinous coating is applied to the entire exposed surfaces of plate 8, whereby plate 8 is effectively isolated from atmospheric conditions and is also mechanically bonded to body 16.

The cross-sectional details of the operative and active portion of the cell hereof are shown in Fig. 7. The energizing currents for the photocell hereof flow, for example, through left pin 14, left silver paste tab 12, left silver paste tab 10 and aquadag bonding tab 11 through the activated lead sulfide layer 9, right silver paste tab 10 and aquadag bonding tab 11, and right silver paste tab 12 back to right pin 14. The effective portion of lead sulfide surface 9 is that located immediately above and between the nearest portions of aquadag bonding tabs 11. The aquadag bonding tabs act as the principal conductors to this active portion, and they also shunt any rectifier junctions produced by the silver paste. This active portion is effectively isolated from ambient humidity by being suspended over resinous mass l3. Likewise, resinous covering 15 also prevents excessive exposure to ambient humidity.

The preferred cell construction shown herein is especially suited for an end-on direct response. That is, this cell is particularly sensitive to radiation emanating from a generally hemispherical volume with the active portion of lead sulfide layer 9 being located at the intersection of the radii of this hemisphere.

, It is to be understood that the above-described arrangement is illustrative of the application of the principles of this invention. Numerous other arrangements may be devised by those skilled in the art without departing from the scope of this invention.

What is claimed is:

l. A photoconductive cell comprising two elongated conductor pins, a solid unitary bead having a disc-shaped circular upper portion and a relatively smaller diameter cylindrical lower supporting portion, said pins being me chanically coupled by said bead so that one of the ends of each of said pins is substantially flush with the upper circlular surface of said disc-shaped portion, a thin rectangular glass plate, a layer of photoconductive material applied to one of the surfaces of said glass plate, resinous means sandwiched between said photoconductive surface and said upper circular surface, electrical conductor means individually interconnecting each of the ends of said glass plate to a different one of said flush pin ends, and second resinous means enveloping and hermetically isolating said glass plate.

2. A photoconductive cell comprising two elongated conductor pins, a solid unitary bead having a disc-shaped circular upper portion and a relatively smaller diameter cylindrical lower supporting potrion, said pins being mechanically coupled by said head so that one of the ends of each of said pins is substantially flush with the upper circular surface of said disc-shaped portion, a thin rectangular glass plate, a layer of photoconductive material applied to one of the surfaces of said glass plate, two electrical conducting tabs each individually applied to different end portions of said photoconductive layer, resinous means sandwiched between the uncovered portion of said photoconductive surface and said upper circular surface, electrical conductor means individually interconnecting each of said conducting tabs to a different one of said flush pin ends, and second resinous means enveloping and hermetically isolating said glass plate.

3. A photoconductive cell comprising two elongated conductor pins, a solid unitary bead having a relatively large flat surface, said pins being mechanically coupled by said head so that one of the ends of each of said pins is substantially flush with said large flat surface, a relatively thin insulating plate which readily transmits infra red radiant energy, a layer of photoconductive material applied to one of the surfaces of said insulating plate, resinous means sandwished between said photoconductive surface and said relatively large flat surface, electrical conductor means individually interconnecting a different portion of said photoconductive layer to a different one of said flush pin ends, and second resinous means enveloping and hermetically isolating said insulating plate.

4. A photoconductive cell comprising two elongated conductor pins, a solid bead, electrical insulating means which readily transmits infra-red radiant energy, a layer of photoconductive material applied to a surface portion of said insulating means, resinous means sandwiched between a portion of said photoconductive surface and a surface portion of said bead, means for establishing electrical connections to said. photoconductive layer, and second resinous means enveloping and hermetically isolating said glass plate.

References Cited in the tile of this patent UNITED STATES PATENTS 

1. A PHOTOCONDUCTIVE CELL COMPRISING TWO ELONGATED CONDUCTOR PINS, A SOLID UNITARY BEAD HAVING A DISC-SHAPED CIRCULAR UPPER PORTION AND A RELATIVELY SMALLER DIAMETER CYLINDRICAL LOWER SUPPORTING PORTION, SAID PINS BEING MECHANICALLY COUPLED BY SAID BEAD SO THAT ONE OF THE ENDS OF EACH OF SAID PINS IS SUBSTANTIALLY FLUSH WITH THE UPPER CIRCULAR SURFACE OF SAID DISC-SHAPED PORTION, A THIN RECTANGULAR GLASS PLATE, A LAYER OF PHOTOCONDUCTIVE MATERIAL APPLIED TO ONE OF THE SURFACES OF SAID GLASS PLATE, RESONOUS MEANS SANDWICHED BETWEEN SAID PHOTOCONDUCTIVE SURFACE AND SAID UPPER CIRCULAR SURFACE, ELECTRICAL CONDUCTOR MEANS INDIVIDUALLY INTERCONNECTING EACH OF THE ENDS OF SAID GLASS PLATE TO A DIFFERENT ONE OF SAID FLUSH PIN ENDS, AND SECOND RESINOUS MEANS ENVELOPING AND HERMITICALLY ISOLATING SAID GLASS PLATE. 